The loss of European funding for medical research could cost the lives of some vulnerable children, London’s Great Ormond Street Hospital (GOSH) has said.
It claims leaving the European Union (EU) poses a “serious risk” to its research funding.
The hospital said it could also lose EU staff and long-standing EU partnerships.
But a leading cancer specialist said the hospital was being “hysterical”.
Angus Dalgleish, professor of oncology at St George’s, University of London, who also represents Brexit movement Scientists for Britain, told the BBC the claims by Great Ormond Street were a “gross overreaction and rather hysterical”.
“There are a lot of scare stories about people not wanting to come because we’re leaving the EU. I don’t think that will happen,” he said.
“There are a lot of us trying to make sure that when Brexit comes, that all the good bits will be left in place. It’s not going to be a break.”
Universities and science minister, Jo Johnson, said the government was fully committed to science and to keeping the UK as “a science powerhouse”.
He said the government would be doing everything it could to maintain that position and “limit some of the risks which the sector has identified.”
“We are more open and outward looking than ever before. We want to forge international collaborations with European partners and countries beyond the EU,” Mr Johnson added.
The Department for Business Innovation and Skills (BIS) said there was no immediate change in funding nor was there likely to be until 2020.
The future of UK access to European research and innovation funding will be a matter for future discussions. But the government was determined to ensure the UK continues to play a leading role in European and international research and innovation, it added.
GOSH called on the government to secure a deal with Europe that protected research and enabled the hospital to “retain its brightest minds” as part of its Brexit negotiations.
Since 2010, GOSH has received £25m in funding for research into new treatments and cures from the EU – equivalent to about 10% of its research budget.
It said it had 44 active EU-funded research projects, with six at clinical trial stage for new treatments.
Fourteen research projects were under review, with “uncertainty about current grants, or applications in the future”, GOSH said.
It argued losing EU funding could “critically delay” the development of revolutionary treatments.
The chief executive of GOSH, Dr Peter Steet, said: “For the vulnerable, very ill children we care for, every day counts.”